These investigations are concerned with mechanisms of gastric acid secretion and mucosal barrier function. Parachloromercuribenzene sulfonate (PCMBS), a relatively impermeable sulfhydryl reagent will be placed on the mucosal surface of exteriorized segments of canine gastric fundus. Ion and water exchange will be assessed, total blood flow will be measured by venous effluent technique while mucosal blood flow will be measured by aminopyrine clearance and distribution of radio-active microspheres. Localization of action on PCMBS will be studied by measuring penetration of a radio-labeled moiety by using radiography. Presence of PCMBS will be determined before and after its displacement from sulfhydryl groups by the use of albumin and cysteine. It is the purpose of these studies to determine if the inhibition of gastric acid secretion seen in the presence of PCMBS is due to an effect on blood flow, mucosal barrier dysfunction, or a combination of the two.